1. Field of the Invention
The present invention relates to an apparatus for imaging an anterior eye part of subject's eye by optical coherence tomography (OCT), in which a tomographic image of subject's anterior eye part is obtained by the OCT and a method therefor.
2. Description of the Related Art
There have conventionally been provided optical coherence tomographic apparatus which serve as an inspection apparatus used for ophthalmic examination by optical coherence tomography (OCT) and obtaining a tomographic image of subject's eye (eyeball) by the OCT. The optical coherence tomographic apparatus includes two types, that is, a time domain type and a Fourier domain type. The time domain OCT apparatus obtains a tomographic image while a mirror is moved so that a light path length of reference light is mechanically changed. The Fourier domain OCT apparatus detects spectral information using a spectroscope in order to obtain a tomographic image. The Fourier domain OCT apparatus may include an optical frequency ramping OCT apparatus which detects a spectral interference signal using a wavelength scanning optical source in order to obtain a tomographic image.
In OCT, subject's eye is generally irradiated with measurement light so that a one-dimensional scanning is carried out, whereby a two-dimensional (2D) tomographic image is obtained (B-scan). A plurality of 2D tomographic images are obtained while a scan line is repeatedly moved relative to the subject's eye, whereby a three-dimensional (3D) tomographic image is obtained (C-scan). FIG. 5A shows a raster scan as one of scanning manners. In the raster scan, a one-dimensional scanning is carried out along a horizontally extending scan line (B-scan). The one-dimensional scanning is repeated while a scan line is vertically moved (C-scan), whereby a 3D image of eyeball is obtained. Consequently, tomographic images along the scan lines can be obtained as shown in FIG. 5B.
FIG. 6A shows a radial scan as another scanning manner. In the radial scan, a one-dimensional scanning is carried out along a radially extending scan line (B-scan). The one-dimensional scanning is repeated while the scan line is circumferentially moved (C-scan). Consequently, tomographic images taken along respective scan lines are obtained as shown in FIG. 6B.
Japanese patent application publication JP-A-2007-127425 discloses a correcting method in optical tomographic imaging method. The document discloses an ophthalmologic photography apparatus having an OCT optical system. Japanese patent application publication JP-A-2006-212153 discloses an ophthalmologic photography apparatus. Japanese patent application publication JP-A-2007-117629 discloses an ophthalmologic imaging apparatus. A 3D image is obtained in each of these documents.
The method and apparatus disclosed by the respective above-mentioned documents each necessitate a relatively longer imaging time period to obtain a 3D image of subject's eye. For example, it takes about 6 seconds to image an anterior eye part with a diameter of 16 mm. Accordingly, the subject has to keep his or her face or eyeball immobile in the meantime, which results in a large burden to the subject. When an object to be measured or subject's eye happens to move during imaging, a precise 3D image cannot be obtained. In particular, a C-scan direction image is apt to be subjected to influences of involuntary eye movement, and accordingly, an obtained tomographic image tends to be inferior in the image quality.
The first and second of the above-mentioned three documents, JP-A-2007-127425 and JP-A-2006-212153 disclose respective methods of correcting the movement of the object to be measured or subject's eye on the obtained tomographic image in a software manner. However, there is a possibility that in the occurrence of blinking or large fixation disparity, the above-described methods may result in an error of correction or display an erroneous image. More specifically, an imaging time needs to be rendered as short as possible in order that a high-precision 3D image may be obtained.
Furthermore, when an anterior eye part is imaged by an OCT imaging apparatus, measurement light is refracted on a substantially spherical cornea (a boundary between the cornea surface and an anterior chamber). As a result, an obtained tomographic image is distorted. Accordingly, the tomographic image refracted on the cornea needs to be corrected. However, since measurement light is incident from a direction differing from a normal direction of the cornea in the conventional B-scan, a tomographic image obtained by one B-scan is affected not only by the refraction on a fault surface thereof but also by the refraction on other surfaces. As a result, the tomographic image needs to be corrected with respect to the refraction on the cornea after a 3D tomographic image has been obtained by execution of C-scan. Since the processing is extremely complicated, the correction is time-consuming and has a difficulty in providing high precision.